Graphene is a single layer of carbon atoms that is the subject of much interest because of its extraordinary electronic and mechanical properties. However, previous methods of synthesis have produced graphene sheets that are either less than 400 nm in length or are multi-layered, wrinkled or crumpled.
First synthesized by mechanical exfoliation in 2004 by Andre Geim and Kostva Novoselov of the University of Manchester, graphene is a 2-dimensional sheet of carbon atoms (˜38 per nm2) bonded in a hexagonal lattice structure. FIG. 1 is a computer-generated model of graphene. Electron motion is only defined in two directions in graphene, meaning it essentially lacks height and is, therefore, 2-dimensional. In its pristine form, graphene is only one atom high with a Van Der Waals' thickness of 0.34 nm. However, due to standard atmospheric wave oscillations and the Mermin-Wagner Theorem, which states that fluctuation in 2-dimensional materials must exist because they favor an increase in entropy, its ideal height becomes 1 nm (±0.3 nm). Graphene's two-dimensional structure gives it mechanical and electrical properties that exceed those of almost all 3-dimensional solid state and some 2-dimensional electron gases.
The theory of monatomic sheets of carbon was developed in 1947 by Phillip Wallace, who claimed that simpler, 2-dimensional objects could display exemplary electronic properties. Because electrons can only travel in two dimensions in graphene, they spend less time “turning corners” and face less resistance from the internal lattice structure. In fact, graphene has the highest electron mobility rating at 15,000 cm2 V−1 s−1 and the lowest electron resistivity ever recorded at 1×10−9 Ωm. Pristine graphene's electrical and thermal conductivities are also reported to be 6 S cm−1 and 5,020 W m−1 K−1 at room temperature, respectively, both of which are almost ten times higher than the respective electrical and thermal conductivities of silver. Graphene's mechanical properties are also exceptional. Its tensile strength of 400,000 MPa is the highest ever recorded. Its surface area of 2,600 m2/g is also exceptionally high.
As of now, graphene's electrical properties, which are far superior to silicon's, make it highly desirable for use in electronic transistors, superconducting switches, and electrochemical supercapacitors. In 2008, researchers from the National University of Singapore tested the contact effects of electrons in graphene transistors. They found that, compared to electrons traveling through other basic metals, electrons traveling through nanoribbons of graphene had little contact with the actual carbon atoms. They also measured the local density of state of graphene. A local density of state measures the number of states, or modes, that are available for varying wavelengths of light to occupy. In short, a local density of state inversely measures the bandgap of a substance, meaning that the smaller the bandgap, the better the conductivity. They found graphene's local density of state to be 12×1019 n/cm3, which is 4 times larger than that of silicon, indicating exceptional conductivity.
Researchers at the IBM TJ Watson Research Center in New York found the Fermi level, which measures the distance (difference) between the valence and conduction band (states in which electrons can exist) at absolute zero, of graphene to be 0.4 eV. This is quite remarkable, considering that the lowest Fermi level of any basic metal is that of Cesium at 1.59 eV. Electron velocity can also be determined by a combination of Fermi level and microscopic Ohm's law. Presumably, the lower the Fermi level and electron resistivity, the faster the electron velocity. Thus, because it has a low resistivity and Fermi level, electrons must travel extremely fast in graphene. Furthermore, they found that under a 0 V drain bias, graphene has an electron carrier concentration of 1.1×1011 cm−2. Under similar conditions, silicon has a carrier concentration of only 1×1010 cm−2.
There have been many different techniques developed in an attempt to synthesize graphene. Park et al. mechanically cleaved graphite by stamping it with an epoxy-coated TEM grid, but this produced multilayers as well as some single-layered areas. Stankovich et al. treated graphene oxide with organic isocyanides and then made graphene-polymer composites, but not pure graphene. Yang et al. functionalized graphene sheets with 1-(3-aminopropyl)-3-methylimidazolium bromide. Yang's “functionalized graphene” is illustrated in FIG. 2. N. Liu et al. electrochemically functionalized graphite immersed in a solution of water and 1-octyl-3-methyl-imidazolium hexafluoropbosphate. TEM images depicted multi-layered graphene and AFM height profiles of 4 nm sheets in some places, much higher than a monatomic carbon layer.
In 2006, Niyogi et al. refluxed graphene (graphitic or graphite) oxide (“GO”) with thionyl chloride and dimethylformamide for 24 hours. The oxidized GO was then reacted with octadecylamine for 4 days, filtered and dispersed in tetrandrofuran. Raman spectra (i.e., a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in a system) demonstrated highly functionalized graphene sheets. Researchers at the Pacific Northwest National Laboratory in Richland, Wash. under Rong Kou synthesized graphene by the rapid thermal expansion to 1200° C. of expandable graphite and found that the resulting sheets were extremely wrinkled, as shown in FIG. 3. Others have also tried this method and obtained similar results. Some even found that after expansion under 1000° C., the graphite did not exfoliate to less than 4 or 5 layers.
Chemical reduction of GO is the method many researchers use as the first step towards graphene synthesis. GO is layered graphene with epoxide and hydroxyl groups on the edges of a 3-dimensional lattice structure, as illustrated in FIG. 4. When GO is reduced, the functional groups on the edges are removed, thus releasing the layered graphene sheets into individual pieces. J. I. Paredes at the National Carbon Institute in Oviedo, Spain synthesized high-quality graphene in 2008. He first used the Hummers method of oxidizing graphite with a mixture of H2SO4, KMnO4, and NaNO3 to produce GO and then reduced it with a 2:1 ratio of ammonia:hydrazine monohydrate, refluxing for 1 hour. This graphene displayed an ideal thickness of 1 nm and was multi-layered in very few places but had, at most, lengths of 300 nm. Researchers under Dan Li used Paredes' method but substituted a 1:7 solution of hydrazine:ammonia and filtered the suspension after reduction. Li's graphene sheets were also the ideal thickness, but comparatively small (200 nm).
In 2007, under Guoxiu Wang, researchers in Australia synthesized graphene by first forming GO through the Hummers method and reducing it by refluxing with hydroquinone for 20 hours. Their graphene sheets were wrinkled, less than 600 nm in length, and multi-layered. Raman results also suggested that the GO had been reduced not to graphene but back to graphite due to the presence of a graphite peak at 2600 cm−1.
Bourlinos et al. made two dispersions of 100 mg of GO in 20 ml water. To one sample, he added 200 mg NaBH4 and heated the mixture in a steam bath for 3 hours, producing graphite. To the other sample, he added 300 mg hydroquinone C6H4(OH)2 and refluxed for 20 hours, also producing a graphitic solid. Wang et al. in Australia suspended GO in an unspecified mixture of ethanol and water and then reduced with hydroquinone by refluxing for 20 hours, centrifuging, washing and drying the precipitate. FIG. 5 shows these graphene sheets, which resembled crumpled silk, entangled and rippled with each other. Chen dispersed GO in H2O and used p-phenylene diamine as a reducing agent, but prepared a chemically modified graphene colloid. Shin et al. dispersed GO in an aqueous solution of NaOH (pH=10) and found that N2H4 reduction produced films with some C—N groups, which can act as n-type chemical dopants, producing relatively higher sheet resistance. Shin also tested reduction by dipping GO films that were deposited onto PET substrates into NaBH4 at 15, 50, and 150 mmolar concentrations. It was found that a large increase in defects, monitored by the ratio of the D:G band intensities of the Raman spectra, was generated by the 150 mmolar concentration, but that the 15 mmolar NaBH4 did not completely remove all the carbonyl groups.
Thus, prior art methods of GO reduction include using sodium borohydride (NaBH4), hydrazine (N2H4) or its hydrate form, and hydroquinone (C6H4(OH)2) have been unable to create a flat, pure graphene sheet greater than 1 μm long. These processes produce individual graphene sheets, but they are crumpled, wrinkled, cracked, or only 200-300 nm long. A typical example of such sheets is shown in FIG. 6. Accordingly, there is a need for a method to synthesize sizable, flat monolayer graphene sheets without functionalizing them.